1. Field of the Invention
The present invention relates to a rear suspension for a vehicle such as an automobile, and more particularly, to a twist beam type rear suspension.
2. Description of the Prior Art
A conventionally general twist beam type rear suspension has such a construction as shown in FIG. 17, wherein it comprises a pair of trailing arms 100R and 100L disposed along the opposite sides of a vehicle and a twist beam 102 extending transversely in the vehicle to be connected between said pair of trailing arms, with front ends of the trailing arms being pivotably supported from a vehicle body 108 via joints 106R and 106L. The joints 106R and 106L generally include cylindrical rubber bushes and define a transverse axis of pivoting 104. The rear ends of the trailing arms support vehicle wheels 110R and 110L, respectively, to be rotatable about respective axes of rotation 128 which are generally in coincidence with one another when the vehicle wheels are at their standard positions. The twist beam 102 is twistable about an axis of twisting 112 determined by the cross sectional shape thereof.
In such a twist beam type rear suspension, when the opposite vehicle wheels 110R and 110L bound or rebound relative to the vehicle body in the same phase as one another, the trailing arms 100R and 100L swing up and down about the axis of pivoting 104, whereas when the opposite vehicle wheels bound or rebound in the phases opposite to one another, the trailing arms 100R and 100L swing about phantom straight lines 118R and 118L, respectively, wherein the phantom straight line 118R is a phantom straight line passing a point of intersection 116 between the axis of twisting 112 and a phantom vertical center plane 114 of the vehicle and the pivot center of the joint 106, while the phantom straight line 118L is a phantom straight line passing said point of intersection 116 and the pivot center of the joint 106L. Therefore, the steering alignment of the suspension changes when the opposite vehicle wheels bound or rebound in the phases opposite to one another. In the opposite phase bounding or rebounding of the vehicle wheels, the twist beam 102 is twisted about the axis of twisting 112, functioning as a stabilizer.
In Japanese Utility Model Laying-open Publication 63-40210, a twist beam type rear suspension is disclosed to have such a construction as shown in FIG. 19. This suspension comprises, in addition to the basic construction of the conventionally general twist beam type rear suspension, a pair of control links 124R and 124L pivotably connected at first ends with the corresponding trailing arms 100R and 100L via joints 120R and 120L and pivotably connected at second ends with the vehicle body 108 via joints 122R and 122L, respectively, with the pivot centers of the joints 122R and 122L being positioned on the axis of pivoting 104, while phantom straight lines 126R and 126L passing the pivot centers of the opposite ends of the control links 124R and 124L, respectively, intersect one another at a point 130 positioned rearward of the axes of rotation 128 of the vehicle wheels as viewed from the top of the vehicle. In this case, the steering alignment of the vehicle wheels changes toward the understeering when a transverse force is applied to the vehicle wheels.
Referring again to FIG. 17 showing the conventionally general twist beam type rear suspension, denoting the distance between the axis of pivoting 104 of the trailing arms 100R and 100L and the axes of rotation 128 of the vehicle wheels by Ly, and denoting the distance between the pivot center of the joint 106R alternatively 106L and the phantom vertical center plane 114 by Lx, when a transverse force Fx is applied to the vehicle wheels 110R and 110L, longitudinal forces Fxr and Fxl are exerted at the joints 106R and 106L, respectively, said longitudinal forces each being of a magnitude of Fx.multidot.Ly/Lx and opposite to one another in the direction.
Due to these longitudinal forces the rubber bushes incorporated in the joints 106R and 106L are deformed, so that the suspension turns as a whole about a center point 132 between the joints 106R and 106L as illustrated by broken lines in FIG. 17, thereby shifting the opposite vehicle wheels 110R and 110L with respect to the steering alignment and the transverse position.
Although not illustrated in FIG. 17, a side force is also exerted to the joints 106R and 106L, so that the rubber bushes in these joints are also elastically deformed in the transverse direction, thereby shifting the suspension as a whole in the transverse direction.
Therefore, in order to improve the steering stability of the vehicle, the spring constant of the rubber bushes in the joints 106R and 106L need to be increased so that the shifting in the steering alignment and the transverse position of the vehicle wheels due to a transverse force applied to the vehicle wheels is minimized.
On the other hand, as shown in FIG. 18, which shows the same suspension as shown in FIG. 17, when a longitudinal force Fy is applied to the vehicle wheels 110R and 110L, longitudinal forces Fyr and Fyl of the same magnitude are exerted in the joints 106R and 106L, respectively. Therefore, the rubber bushes in the joints 106R and 106L are elastically deformed due to these longitudinal forces, so that the suspension is shifted as a whole in the longitudinal direction as illustrated by broken lines in FIG. 18.
It is generally required that a relatively large longitudinal compliance is available in the suspension for improving the riding comfortableness of the vehicle. Therefore, in order to improve the riding comfortableness in the conventionally general twist beam type rear suspension, the spring constant of the rubber bushes in the joints at the front ends of the trailing arms need to be decreased.
Therefore, in the conventionally general twist beam type rear suspension there is an inconsistency in that the rubber bushes in the joints at the front ends of the trailing arms are to be harder from the view point of improving the steering stability of the vehicle, while they are to be softer from the view point of improving the riding comfortableness of the vehicle.
In the twist beam type rear suspension described in the above-mentioned Japanese Utility Model Laying-open Publication 63-40210, a transverse force applied to the vehicle wheels shifts the steering alignment toward the understeering, as the instant center of turning of the suspension as a whole is positioned at the point of intersection 130 of the axes of the pair of control links 124R and 124L, contributing to the steering stability of the vehicle. Or, if the point of intersection 130 is positioned on the axes of rotation of the vehicle wheels as viewed from the top of the vehicle, the transverse rigidity of the suspension is improved.
However, when a longitudinal force is applied to the vehicle wheels, the opposite control links are pulled or compressed in the same way, and therefore, the suspension can not shift longitudinally, if the joints 120R, 120L, 122R and 122L are constructed to be hard enough to provide the above-mentioned improvement in the steering stability of the suspension, and therefore, no longitudinal compliance is available in the suspension, and no improvement of the riding comfortableness is available. This situation is not compensated for by the condition that the point of intersection 130 is positioned rearward of the axes of rotation 128 of the vehicle wheels as shown in FIG. 19.
Further, in this construction, when the vehicle wheels bound or rebound in the phases opposite to one another, the trailing arms 100R and 100L would swing about the phantom straight lines 118R and 118L, respectively. However, the control links 124R and 124L connected with the trailing arms 100R and 100L at the joints 120R and 120L and with the vehicle body 108 at the joints 120R and 120L, respectively, not positioned on the phantom straight line 118R or 118L, will restrict the swinging movement the trailing arms 100R and 100L, generating the so called link interference.